Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images which show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions which emanate from the body and are captured by a scintillation crystal, with which the photons interact to produce flashes of light or “events.” Events are detected by an array of photodetectors, such as photomultiplier tubes, and their spatial locations or positions are calculated and stored. In this way, an image of the organ or tissue under study is created from detection of the distribution of the radioisotopes in the body.
One particular nuclear medicine imaging technique is known as Positron Emission Tomography, or PET. PET is used to produce images for diagnosing the biochemistry or physiology of a specific organ, tumor or other metabolically active site. Measurement of the tissue concentration of a positron emitting radionuclide is based on coincidence detection of the two gamma photons arising from positron annihilation. When a positron is annihilated by an electron, two 511 keV gamma photons are simultaneously produced and travel in approximately opposite directions. Gamma photons produced by an annihilation event can be detected by a pair of oppositely disposed radiation detectors capable of producing a signal in response to the interaction of the gamma photons with a scintillation crystal. Annihilation events are typically identified by a time coincidence between the detection of the two 511 keV gamma photons in the two oppositely disposed detectors, i.e., the gamma photon emissions are detected virtually simultaneously by each detector. When two oppositely disposed gamma photons each strike an oppositely disposed detector to produce a time coincidence event, they also identify a line of response, or LOR, along which the annihilation event has occurred.
An example of a PET method and apparatus is described in U.S. Pat. No. 6,858,847, which patent is incorporated herein by reference in its entirety. After being sorted into parallel projections, the LORs defined by the coincidence events are used to reconstruct a three-dimensional distribution of the positron-emitting radionuclide within the patient. PET is particularly useful in obtaining images that reveal bioprocesses, e.g. the functioning of bodily organs such as the heart, brain, lungs, etc. and bodily tissues and structures such as the circulatory system.
Depending upon the isotope used, the spatial resolution obtainable with PET imaging is currently limited to 1-8 mm. For example, positrons emitted from 18F used in FDG (a common radiopharmaceutical used in PET imaging) obtain a resolution of approximately 1 mm FWTM (Full Width at Tenth Maximum) in water, and positrons emitted from 15O obtain a resolution of approximately 4 mm FWTM (Full Width at Tenth Maximum) in water. Current commercially available small animal PET systems are able to image close to 1 mm spatial resolution FWTM (Full Width Half Maximum). Some academic proof-of-principle PET detectors are known which can image close to 600 μm; however in real world applications the detectors would not be able to achieve such resolution without limiting positron range.
PET technology advances have included finer detector elements designed to improve spatial resolution. However, there is a limit to what extent reducing detector element size will improve spatial resolution in PET. The spatial resolution of PET imaging is limited by several other factors, such as annihilation photon non-colinearity, positron range, off-axis detector penetration, detector Compton scatter, undersampling of the signal in the linear or angular directions for the image reconstruction process, and patient motion. The overall spatial resolution of the systems is a convolution of these components. Of these other factors that contribute to resolution broadening, perhaps the most uncertain and, for certain isotopes, the most dominant effect is from positron range. Positron range refers to the distance that a positron travels after emission and prior to annihilation by an electron.
In the paper entitled “Combined MRI-PET Scanner: A Monte Carlo Evaluation of the Improvements in PET Resolution Due to the Effects of a Static Homogeneous Magnetic Field,” IEEE Transactions on Nuclear Science, Vol. 43, No. 4, August 1996, Raylman et al. simulated the potential improvement to spatial resolution of a PET detector operating inside a static homogeneous magnetic field associated with a magnetic resonance scanner.
Further, U.S. Pat. No. 4,939,464 to Hammer, incorporated herein by reference in its entirety, discloses a combination NMR-PET apparatus wherein a PET detector is located inside a cylindrical magnet of a nuclear magnetic resonance scanner. The '464 patent discloses that in the presence of a magnetic field a positron will not maintain a circular orbit but will spiral towards the center of the orbit, with the result that in the presence of a magnetic field positrons emitted transverse or perpendicular to the magnetic field will be confined to a range defined by the mass, velocity and charge of the positron and the magnetic field.
There remains a need in the art for improvement in spatial resolution of a PET imaging system without combination with a MRI scanner, and beyond the improved resolution observed when a PET system is embedded within the one-dimensional magnetic field of a MRI scanner.